Engines, including diesel engines, gasoline engines, gaseous fuel-driven engines, and other engines known in the art, traditionally exhaust a complex mixture of pollutants. These emissions may include gaseous and solid material, such as, particulate matter, nitrogen oxides (NOx), and sulfur compounds. Heightened environmental concerns have led regulatory agencies to increase the stringency of emission standards for such engines, forcing engine manufactures to develop systems to further reduce levels of engine emissions.
One method used by engine manufacturers to reduce engine emissions includes exhaust gas recirculation (EGR). EGR systems recirculate a portion of the engine exhaust stream into the air-intake supply of the engine to reduce oxygen concentration within a combustion chamber. Recirculated exhaust gas may further act to lower combustion temperatures, and in combination with reduced oxygen concentration, may lead to reduced emission levels.
Selective catalytic reduction (SCR) provides another method for reducing engine emission levels. During SCR, a catalyst facilitates a reaction whereby ammonia and NOx react in an exhaust stream to produce water and nitrogen gas, thereby reducing NOx levels in the exhaust gas. Generally, the ammonia used for the SCR system may be either produced during engine operation or stored for injection as required. However, because of the high reactivity of ammonia, storage of ammonia can be hazardous. Further, on-board production of ammonia can be costly and may require specialized equipment.
One system configured to reduce emission levels with an EGR system is described in U.S. Pat. No. 6,286,489 (“the '489 patent”) issued on Sep. 11, 2001 to Bailey. The '489 patent describes an EGR system in which a portion of the exhaust gas from a first cylinder group is cooled by an exhaust gas cooler before being directed to an intake manifold of an engine. The system further includes a valve configured to adjust a flow rate of exhaust gas from the first cylinder group to the air intake manifold.
While the system of the '489 patent may reduce NOx from an exhaust stream through use of EGR, the system can be further improved. In particular, some engines may operate under conditions whereby an EGR system may not provide sufficient emission reduction. Such engines may benefit by having an additional system configured to further reduce emission levels, such as an SCR system. However, the engine of the '489 patent includes only an EGR system and is not configured to operate with an SCR system that may further reduce emissions.
The present disclosure is directed at overcoming one or more of the limitations in the prior art.